Methods for analyzing respirable particles in bulk materials

ABSTRACT

Provided is a method for detecting respirable participles in a bulk material comprising particles. The method comprises: analyzing morphology of the particles; analyzing chemical composition of the particles; creating a profile of the particles, wherein each particle in the profile is characterized by its shape, size and chemical composition; selecting particles from the profile which match the size and chemical composition of a respirable particle; and calculating a percentage of the respirable particles in the bulk material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from US Provisional Patent Application 62/361,273 filed on Jul. 12, 2016, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention provides methods for accurate and rapid detection of respirable particles in bulk materials.

BACKGROUND

Many compounds used by the building construction industry are bulk materials, including, but not limited to, gypsum, calcined gypsum, mica, cement, calcium carbonate and sand. These dry compounds comprise a population of particles of different sizes.

Particles which are smaller than 20 microns are referred to respirable particles because these particles can be dispersed into the air. These particles can be then inhaled by workers, which should be avoided. Thus, there is a need to analyze a powder sample and provide an accurate estimate whether the sample comprises respirable particles and the percentage of such particles in the sample.

The standard procedure for respirable silica requires the sample in analysis to be treated with acid to eliminate most of the sample matrix. The remaining material is examined for crystalline silica. The weight of crystalline silica is determined using X-ray Diffraction or FTIR. This number is reported as Total Crystalline Silica. The second step requires the sample to be dispersed in alcohol and transferred to a silver membrane and the mass of crystalline silica determined for the <10 μ fraction. A determination is also provided for the <5 μ fraction.

The criteria in the Globally Harmonized System (GHS) have the fine fraction of silica classified as specific target organ toxicity in this case, the lung. Generic cut-off values for products containing a fine fraction of crystalline silica trigger a need for a method for the quantification of the fine fraction of crystalline silica in bulk materials.

The European Union has promoted an analysis for silica in bulk materials which culminated in the publication of a new standard for measuring the amounts of respirable particles in bulk materials. The standard has two parts:

1. Determination of Size-Weighted Potential Respirable Fraction (SWeRF); and

2. Size-Weighted Potential Respirable Fraction of Crystalline Silica (SWeRF_(CS))

The method is to be used for comparing the potential health risks of bulk materials. The method does not predict how a material will disperse in air, but quantifies the respirable fraction. The particles that present a larger health risk are weighed more in the calculation. The advantage of the method is it provides an unambiguous characterization of the bulk material. The term “potential” is used to indicate that the standard does not analyze airborne particles.

The standard describes a method using sedimentation and a calculation method based on particle size distribution (PSD). The calculation can only be used after the results are validated using the sedimentation data. The calculation method requires that the particles have the same density or in the case of mixture with different materials that they have the same PSD. A plot is made to compare the sedimentation PSD with the Stokes' Law and the convention described in CSN EN 481 (European Standards EN 481 “Workplace atmospheres—size fraction definitions for measurement of airborne particles”). However, in this calculation method, the dynamic form factor is neglected where in the sedimentation method, the dynamic form factor is assumed to be equal in air and liquid. Thus, there remains a need for accurate and rapid detection of respirable particles in a bulk material.

SUMMARY

At least some of these needs are addressed by the present methods which are based on analyzing particles for their morphology and chemical composition and creating a profile for a sample in which each particle is characterized by its morphology and chemical composition.

This invention provides a method for detecting respirable participles in a bulk material comprising particles. The method comprises:

-   -   analyzing morphology of the particles;     -   analyzing chemical composition of the particles;     -   creating a profile of the particles, wherein each particle in         the profile is characterized by its shape, size and chemical         composition;     -   selecting particles from the profile which match the size and         chemical composition of a respirable particle; and     -   calculating a percentage of the respirable particles in the bulk         material.

In this method, the morphology and chemical composition of the particles can be analyzed by a computer-controlled scanning electron microscope interfaced with an energy dispersive X-ray spectrometer (SEM□EDS). The method may comprise a step of resuspending the particles of the bulk material, such as gypsum or calcium carbonate, in a medium (water or organic solvent), and filtering the suspension through a filter with a nominal pore size sufficiently small to retain the particles in the respirable size range. In the method, the particles can be retained on a filter. The morphology and chemical composition of the particles are analyzed by an automated SEM□EDS. Various bulk materials can be analyzed, including mixtures of inorganic compounds.

The present method can be used to analyze respirable particles, such as for example silica (SiO₂), smaller than 20 microns. The present method can be also used to analyze respirable particles, such as for example silica (SiO₂), smaller than 10 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of methods for respirable particle analysis.

FIG. 2A is a plot for a bulk material comprising a small respirable faction.

FIG. 2B are calculations in connection with the plot of FIG. 2A.

FIG. 3A is a plot for bulk material which does not comprise detectable respirable silica.

FIG. 3B are calculations in connection with the plot of FIG. 3A.

FIG. 4A is a plot of a bulk material comprising a respirable fraction.

FIG. 4B are calculations in connection with the plot of FIG. 4A.

FIG. 5 is a scan of gypsum spiked with 0.5% of quartz.

FIG. 6 is a scan of gypsum spiked with 0.1% of quartz.

FIG. 7 is a plot with three calibration points.

FIG. 8 is a scan of selenite comprising quartz.

FIG. 9 is a first plot of filler Microwhite 100.

FIG. 10 is a first plot of filler Pulpro-20.

FIG. 11 is a pot of filler #3.

FIG. 12 is a second plot of filler Microwhite 100.

FIG. 13 is a second plot of filler Pulpro-20.

FIG. 14 is a plot of filler Snowhite 21.

FIG. 15 is a plot of filler S-200.

FIG. 16 is a plot of filler Marblehill.

FIG. 17 is a plot of filler G260 RM 71778.

FIG. 18 is an example of ESPRIT's Feature analysis workspace: a fraction of the filter area is imaged by SEM and converted to binary image, in which particles of interest (bright feature in the left frame) are identified for further EDS analysis (right frame).

FIG. 19 is an example of automated particle analysis workspace: EDS composition analyses of particles marked in FIG. 18 are in progress; note that results from particle sizing are tabulated below the field image.

FIG. 20 is the particle analysis workspace: an example EDS X-ray spectrum acquired for one particle (most likely dolomite) identified in FIG. 18; note that results from EDS are tabulated below the spectrum.

FIG. 21 is a histogram of size (as AED) frequency of particles with Si relative intensity >5% (cf. Table 4).

FIG. 22 reports Si contents in particles (with relative intensity >% 5) as a function of size.

DETAILED DESCRIPTION

Provided is a method for examining the respirable fraction in bulk materials. The method comprises the following two steps. In step one, a sample is analyzed for particle size distribution and density. This step determines the total proportion of respirable particles in the sample. In step two, the amount of crystalline silica (also known as silicon dioxide or quartz) in the sample is determined. Various matrices can be analyzed by this method, including gypsum, cement, mica, calcium carbonate, sand, etc. The method is illustrated in FIG. 1 in comparison to other methods.

Step one can be performed by using a Quantachrome Pycnometer for a density measurement, followed by an analysis with a particle size analyzer (such as for example, Horiba LA-950V2) and the SWeRF equation which is validated for individual matrices. In step two, the mass of silica in the respirable fraction can be estimated by using a scanning electron microscope which eliminates the need for X-Ray Diffraction Instrument. In further embodiments, a scanning electron microscope interfaced with an energy dispersive X-ray spectrometer can be used to analyze particles for their morphology, including shape and size, and also a chemical (elemental composition) of the particles. See FIG. 1 in which the present method is compared to other methods.

In the present method, a scanning electron microscope is used for crystalline identification and morphology. This analysis can be conducted with computer software which captures data for each particle individually, including the particle's shape, size and chemical composition.

In one embodiment of the method, the SWeRF equations are evaluated using the CIC particle size analysis and RJ Lee Total Crystalline Silica. A typical graphic output for plotting the PSD for the SWeRF calculation is shown in FIG. 2A. The plot indicates the DuPont Richmond FGD (trimodal PSD) has a small respirable fraction (1.6%). Using the SWeRF and SWeRF_(CS) equations as shown in the Table of FIG. 2B, the RJ Lee and SWeRF_(CS) estimate are in agreement and report 0.01% respirable silica.

As shown in FIG. 3A and the Table of FIG. 3B, the Dayton Power Light FGD illustrates a scenario where the PSD and SWeRF equations indicate no respirable fraction (monomodal PSD) and therefore no respirable silica. The RJ Lee data is indicating total crystalline silica of 0.2% and <10 μ respirable silica of 0.2%.

As shown in FIG. 4A and the Table of FIG. 4B, the Montreal Recycle material illustrates a scenario where the PSD (bimodal) and SWeRF equations indicate a small respirable fraction in contrast to the RJ Lee data indicating a respirable silica fraction of 1.3%. This is significantly higher than the 0.8% SWeRF estimate. This could require the sedimentation verification for SWeRF.

Table 1 is a summary of several different types of raw materials and how the SWeRF estimates correlate with the RJ Lee respirable silica.

As shown in Table 1, in some examples (Dayton Power FGD, Rodemacher Fly Ash) there are two outputs. This occurs when the SDS gives a range for the density. For example, the Rodemacher SDS provided a density range of 2200-2800 kg/m³. The calculation for SWeRF was performed twice (2200 and 2800 kg/m³).

The XRD experiment on gypsum spiked with 0.5% and 0.1% quartz shows excellent initial sensitivity, as shown in FIGS. 5 and 6. Even though this only is three calibration points (FIG. 7), a scan of selenite indicates the material is 0.056% quartz (FIG. 8).

Various carbonates which can be used as a filler were tested for SWeRF and SWeRFcs by the method described above. These values are provided in Table 2 below. See also FIGS. 9-17 for supporting plots.

TABLE 2 Estimated Respirable Fractions SWeRF = SWeRFcs = 2016-148 #1 80388-CP Filler 7.34 0.09 2016-148 #2 Marblewhite 310 8.30 0.030 2016-148 #3 70283-BP-U 7.02 0.052 2016-159 #1 62303-Microwhite 100 Sylacauga 9.63 0.12 2016-159 #2 66342-Pulpro-20 6.69 0.37 2016-159 #3 63305-Snowhite 21 8.73 0.33 2016-159 #4 70999 S-200 8.21 0.020 2016-159 #5 60208 Microwhite 100 Marblehill 9.07 0.027 2016-182 #1 G260 RM 71778 7.75 0.024 Table 3 lists the particle size distribution for the carbonates.

TABLE 3 SMI Marble- Imerys Omya white CP-Filler BP-LU VOLUME DENSITY 310 VOLUME DENSITY VOLUME DENSITY RUN (cc) (g/cc) RUN (cc) (g/cc) RUN (cc) (g/cc) 1 0.3353 2.9871 1 0.3569 2.8116 1 0.3603 2.7781 2 0.3332 3.0063 2 0.4005 2.5052 2 0.3588 2.7903 3 0.3366 2.9757 3 0.3612 2.7779 3 0.3603 2.7789 4 0.3354 2.9858 4 0.3621 2.7711 4 0.3639 2.7507 5 0.3392 2.9528 5 0.3642 2.7551 5 0.3667 2.7299 6 0.3384 2.9596 6 0.3646 2.7523 6 0.3689 2.7135 7 0.3397 2.9485 7 0.3659 2.7420 7 0.3707 2.7005 8 0.3391 2.9534 8 0.3257 3.0811 8 0.3727 0.6863 9 0.3413 2.9347 9 0.3703 2.7096 9 0.3745 2.6734 10 0.3405 2.9414 10 0.3700 2.7121 10 0.3722 2.6894 average 0.3379 2.9645 average 0.3641 2.7618 average 0.3669 2.7291 std. dev. 0.0026 0.0231 std. dev. 0.0181 0.1400 std. dev. 0.0058 0.0430 Imerys Imerys Microwhite Microwhite 100- 100- Huber Sylacauga VOLUME DENSITY Marblehill VOLUME DENSITY G260 VOLUME DENSITY RUN (cc) (g/cc) RUN (cc) (g/cc) RUN (cc) (g/cc) 1 0.3666 2.7324 1 0.3694 2.7272 1 0.3683 2.7211 2 0.3670 2.7295 2 0.3711 2.7141 2 0.3689 2.7172 3 0.3700 2.7073 3 0.3741 2.6927 3 0.3691 2.7159 4 0.3702 2.7058 4 0.3768 2.6735 4 0.3699 2.7098 5 0.3705 2.7042 5 0.3789 2.6587 5 0.3688 2.7174 6 0.3731 2.6853 6 0.3804 2.6483 6 0.3714 2.6985 7 0.3731 2.6849 7 0.3822 2.6354 7 0.3682 2.7220 8 0.3742 2.6775 8 0.3815 2.6406 8 0.3709 2.7026 9 0.3729 2.6864 9 0.3820 2.6370 9 0.3675 2.7274 10 0.3744 2.6759 10 0.3789 2.6582 10 0.3683 2.7215 average 0.3712 2.6989 average 0.3775 2.6686 average 0.3691 2.7153 std. dev. 0.0028 0.0204 std. dev. 0.0046 0.0327 std. dev. 0.0012 0.0091 Omya JaJack Snowhite Omya S-200 VOLUME DENSITY 21 VOLUME DENSITY Pulpro 20 VOLUME DENSITY RUN (cc) (g/cc) RUN (cc) (g/cc) RUN (cc) (g/cc) 1 0.3581 2.7958 1 0.3615 2.8165 1 0.3596 2.8497 2 0.3572 2.8031 2 0.3609 2.8212 2 0.3555 2.8826 3 0.3595 2.7851 3 0.3643 2.7949 3 0.3583 2.8598 4 0.3594 2.7854 4 0.3647 2.7913 4 0.3713 2.7598 5 0.3631 2.7573 5 0.3676 2.7693 5 0.3574 2.8677 6 0.3618 2.7672 6 0.3680 2.7663 6 0.3593 2.8518 7 0.3607 2.7758 7 0.3701 2.7512 7 0.3594 2.8513 8 0.3597 2.7831 8 0.3701 2.7508 8 0.3649 2.8083 9 0.3630 2.7583 9 0.3694 2.7562 9 0.3597 2.8487 10 0.3619 2.7668 10 0.3714 2.7415 10 0.3619 2.8320 average 0.3604 2.7778 average 0.3668 2.7759 average 0.3607 2.8412 std. dev. 0.0020 0.0154 std. dev. 0.0037 0.0284 std. dev. 0.0045 0.0348 FIGS. 9-17 provide plots supporting the data in Table 3.

The data in Table 3 indicate that 5 carbonates out of all carbonates listed in Table 2 meet the new silica limit. A person of skill will also readily understand that the final contribution to the product of the respirable fraction depends on the formulation level.

Further embodiments provide a method in which particles are analyzed individually to measure concentrations of respirable particles (such as for example, silica, silcate minerals, asbestos, and any other particles that may be hazardous to a human if inhaled) in a bulk material for safety assessment.

In this method, a sample of bulk material is dispersed and resuspended in a suitable medium. The sample can be resuspended in water or in an organic solvent, including, but not limited to, isopropanol or ethanol. The choice of a medium depends on the water solubility for a particular material to be analyzed. For materials soluble in water, an organic solvent is used.

A bulk material suspended in a medium can be subjected to filtration through a membrane filter with pore sizes suitable for retaining particles in the respirable size range. The respirable size range can be less than 20 μm in some applications, whereas in other applications, it can be less than 10 μm. This method can be performed with 0.4-μm pore-sized polycarbonate filter to ensure all particles in the respirable size range are captured.

In one application of the present method, particles retained on the membrane filter by filtration are air-dried and coated with a thin layer of carbon before being subjected to analysis.

In the present method, an analysis of respirable particles is conducted by a scanning electron microscope (abbreviated as SEM) interfaced with an energy dispersive X-ray spectrometer (abbreviated as EDS). In this analysis, individual particles in a sample are analyzed for two different properties:

a) morphological characteristics, including the particle's size and shape; and

b) chemical (elemental) composition.

One suitable instrumental setup for the present method includes a computer-controlled scanning electron microscope (SEM) interfaced with an energy dispersive X-ray spectrometer (EDS). This technique can be used to obtain accurate morphological (size, shape, etc.) and (elemental) compositional characterizations of thousands of individual particles.

Morphology filters can be used to select a subset of detected particles for further EDS compositional analyses—only particles in the respirable size range (e.g., <10 μm) can be selected if needed. This allows an adequate sampling and counting of deposited particles on the filter in a time-efficient manner.

Results of this analysis are shown in FIGS. 18□20. FIG. 18 depicts a fraction of the filter area imaged by SEM and converted to a binary image, in which particles of interest (bright feature in the left frame) are identified for further EDS analysis (right frame).

FIG. 19 provides the EDS composition analysis of particles marked in FIG. 18. The results from particle sizing are tabulated below the field image in FIG. 19.

FIG. 20 is an example of the EDS X-ray spectrum acquired for one particle (most likely dolomite) identified in FIG. 18. The results from the EDS analysis are tabulated below the spectrum in FIG. 20.

In the present method, a large number of particles can be accurately analyzed for each particle's morphology and chemical composition. The present method creates a multi-dimensional raw dataset (or a profile) of the particles for the sample. In this method, each particle is characterized by its morphological parameters (such as shape and size) and chemical composition.

In one application of the present method, a sample comprising 10% (by weight) respirable silica mixed in gypsum is analyzed for morphology and chemical composition of individual particles. A profile for this sample is shown in Table 4 below. As can be seen from Table 4, the raw dataset may include the following morphological characteristics of a particle: the area, aspect ratio, volume and diameter. In addition, the particle's chemical characteristics are represented by its elemental composition. This is particularly important for identifying different particle types (e.g., gypsum, quartz, calcium carbonate). In this example, each particle was analyzed for the presence of calcium (Ca), sulfur (S) and silicon (Si).

TABLE 4 Selected output parameters from a computer-controlled SEM-EDS analysis, for a sample made with known quantities of gypsum and respirable silica mixed together. Area Volume (μm³)^(b) Equivalent Relative Elemental Particle Area Aspect Prolate Diameter X-ray Intensity^(c) ID (μm²) Ratio^(a) Sphere Spheroid (μm) Ca % S % Si % P4939 2.60 1.63 9.93 2.09 1.82 28.59 25.13 46.28 P4940 0.23 1.74 0.26 0.05 0.54 5.51 6.09 88.40 P4969 1.33 1.82 3.64 0.76 1.30 11.06 11.18 77.76 P4973 3.20 1.56 13.54 2.72 2.02 24.57 21.71 53.72 P4975 2.01 1.40 6.71 1.70 1.60 44.10 43.48 12.42 P4979 0.17 1.52 0.16 0.03 0.46 37.26 42.31 20.43 P4990 0.15 1.35 0.13 0.03 0.43 20.18 17.71 62.11 P4998 0.45 1.17 0.72 0.19 0.76 44.35 44.08 11.57 P5008 0.30 1.42 0.40 0.09 0.62 9.42 7.65 82.93 P5011 0.43 1.86 0.67 0.14 0.74 15.32 15.67 69.02 P5020 1.11 1.48 2.77 0.65 1.19 6.84 5.40 87.76 P5024 8.51 1.41 58.72 11.18 3.29 11.26 10.60 78.14 P5025 0.18 1.23 0.18 0.04 0.48 14.14 14.23 71.63 P5031 0.37 1.85 0.53 0.10 0.68 18.05 18.92 63.03 P5036 0.36 1.28 0.50 0.12 0.67 32.83 30.99 36.18 P5043 0.30 1.84 0.40 0.08 0.62 13.98 15.15 70.87 P5047 0.14 1.46 0.12 0.03 0.42 17.67 16.55 65.78 P5058 0.20 1.68 0.21 0.05 0.50 42.49 42.56 14.95 P5073 0.16 1.58 0.15 0.03 0.45 12.31 10.66 77.03 P5076 0.49 1.75 0.82 0.15 0.79 9.93 10.71 79.36 P5090 10.24 1.95 77.40 15.98 3.61 47.22 41.82 10.95 ^(a)Aspect ratio = length/width, dimensionless; a measure of sphericity of a particle-1 for spheres, <1 for irregular particles. ^(b)Particle volumes estimated for two different shapes (later to be used for mass approximation with density values for different composition. ^(c)Only relevant elements are shown; due to the use of polycarbonate filter membrane for sample preparation, oxygen (O) was not included in semi-quantification, and gypsum was represented by the elemental presence of Ca and S. Silica was represented by Si.

The large amount of information obtained for morphology and chemical makeup of particles affords a plethora of ways in which the raw data can be analyzed. The following example (FIG. 21) presents the frequency distribution of sizes (as area equivalent diameter, AED) of particles with values of Si X-ray relative intensity >5%, for the same sample as in Table 4. Another way to present these data is to show the Si contents in particles as a function of particle size (FIG. 22).

The present method may also include a step of further characterizing and grouping particles according to a combination of particle characteristics listed (but not limited to) in Table 4. For example, normalized X-ray counts (net counts divided by particle size) can be used to differentiate crystalline and amorphous SiO₂.

Additional data analyses can be conducted with a specifically complied code program to selectively group particles according to their characteristics, including shape, size, chemical composition and any combinations of parameters from the raw dataset.

The invention will be now described in more detail by the following non-limiting Examples.

Example 1

A small amount of weighed bulk material was well-mixed and suspended in known volume (50-100 ml) of deionized water or isopropanol depending on the water solubility of the material. An aliquot (generally <10 ml) was pressure filtered through 25-mm diameter, 0.4-μm pore-sized polycarbonate filter. Particles deposited on the membrane filter were air-dried and coated with a thin layer of carbon before being subject to instrument analysis.

An automated, computer-controlled particle analysis was conducted by a scanning electron microscope (SEM) interfaced with an energy dispersive X-ray spectrometer (EDS). The automated particle analysis was used to provide morphological (size, shape, etc.) and (elemental) compositional characterizations of individual particles, results of which are shown in FIGS. 18-20. Morphology filters were used to select a subset of detected particles for further EDS compositional analyses. In this example, only particles in the respirable size range (<10 μm) were selected.

Example 2

A sample comprising 10% (mass concentration) respirable silica mixed in gypsum (micronizing mill was used to homogenize the mixture) was prepared. 8.4 mg of the mixture was suspended in 50 ml isopropanol; and 3 ml of the suspension was filtered through 25-mm diameter, 0.4-μm pore-sized polycarbonate filter, resulting in 0.504 mg of material retained on a deposition area of 3.14 cm².

The sample was analyzed as described in Example 1 and the particle analysis data was tabulated in Table 4 and were also presented in FIGS. 21 and 22.

In this analysis, particles containing ≧10% Si (relative intensity) and ≦10 μm were included in the calculation as respirable silica, and density value of 2.65 g/cm³ was used to estimate a particle mass, assuming a prolate spheroid particle shape. Knowing the fraction of the deposition area analyzed by SEM-EDS, the mass concentration of respirable silica determined by the analysis was 9.8%, which is consistent with that of the prepared sample mixture. 

What is claimed is:
 1. A method for detecting respirable participles in a bulk material comprising particles, the method comprising: analyzing morphology of the particles; analyzing chemical composition of the particles; creating a profile of the particles, wherein each particle in the profile is characterized by its shape, size and chemical composition; selecting particles from the profile which match the size and chemical composition of a respirable particle; and calculating a percentage of the respirable particles in the bulk material.
 2. The method of claim 1, wherein the morphology and chemical composition of the particles are analyzed by a scanning electron microscope interfaced with an energy dispersive X-ray spectrometer.
 3. The method of claim 1, wherein the method comprises a step of resuspending the particles of the bulk material in a medium selected from water and/or organic solvent, and filtering the suspension through a filter with a nominal pore size sufficiently small to retain the particles in the respirable size range.
 4. The method of claim 1, wherein the particles are retained on a filter, and wherein the morphology and chemical composition of the particles are analyzed by a scanning electron microscope interfaced with an energy dispersive X-ray spectrometer.
 5. The method of claim 1, wherein the bulk material is a mixture of inorganic compounds.
 6. The method of claim 1, wherein the respirable particles are smaller than 20 microns.
 7. The method of claim 1, wherein the respirable particles are smaller than 10 microns.
 8. The method of claim 1, wherein the bulk material is selected from the group consisting of gypsum and calcium carbonate.
 9. The method of claim 3, wherein the filter is a polycarbonate filter.
 10. The method of claim 1, wherein the respirable particles comprise silica (SiO₂). 